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Abstract:

A tin oxide particle having at least two diffraction peaks at 2θ
(deg) of 9±1° and 28±1° in XRD measurement by
Cu/Kα radiation. The tin oxide particle preferably shows
diffraction peaks at 2θ (deg) of 19±1°, 48±1°,
and 59±1°. The tin oxide particle preferably has
electroconductivity. The tin oxide particle is preferably produced by
mixing an aqueous solution containing tin (II) and a hydroxyl-containing
organic compound in a heated condition with an alkali.

Claims:

1. A tin oxide particle having at least two diffraction peaks at 2.theta.
(deg) of 9.+-.1.degree. and 28.+-.1.degree. in XRD measurement by
Cu/Kα radiation.

2. The tin oxide particle according to claim 1, which has
electroconductivity.

3. The tin oxide particle according to claim 1, further having
diffraction peaks at 2.theta. (deg) of 19.+-.1.degree., 48.+-.1.degree.,
and 59.+-.1.degree..

4. The tin oxide particle according to claim 1, wherein the diffraction
peaks are attributed to systematic reflection on specific crystal planes
of the tin oxide, and the spacing of crystal lattice planes corresponding
to the first order systematic reflection is 0.94 to 0.95 nm.

5. The tin oxide particle according to claim 1, being substantially free
from a dopant element.

6. The tin oxide particle according to claim 1, which is for use as a
negative electrode active material or a raw material of a positive
electrode active material for lithium secondary batteries.

7. A transparent dispersion comprising water or an organic solvent, and
the tin oxide particle according to claim 1 dispersed in water or the
organic solvent.

8. A process for producing the tin oxide particle according to claim 1,
comprising mixing an aqueous solution containing tin (II) and an organic
compound having a hydroxyl group in a heated condition with an alkali.

9. The process according to claim 8, wherein the organic compound having
a hydroxyl group is a polyvinyl alcohol, a polyol, or a monohydric lower
alcohol.

10. The process according to claim 8, further comprising adding hydrogen
peroxide to the mixture after mixing the alkali.

11. A process for producing the tin oxide particle according to claim 1,
comprising mixing an alkali into an aqueous solution containing tin (II)
in a heated condition in such an amount as to produce a molar quantity of
OH.sup.- 0.1 to 1.6 times the molar quantity of the tin (II).

12. The tin oxide particle according to claim 2, further having
diffraction peaks at 2.theta. (deg) of 19.+-.1.degree., 48.+-.1.degree.,
and 59.+-.1.degree..

13. The tin oxide particle according to claim 2, wherein the diffraction
peaks are attributed to systematic reflection on specific crystal planes
of the tin oxide, and the spacing of crystal lattice planes corresponding
to the first order systematic reflection is 0.94 to 0.95 nm.

14. The tin oxide particle according to claim 3, wherein the diffraction
peaks are attributed to systematic reflection on specific crystal planes
of the tin oxide, and the spacing of crystal lattice planes corresponding
to the first order systematic reflection is 0.94 to 0.95 nm.

15. The tin oxide particle according to claim 2, being substantially free
from a dopant element.

16. The tin oxide particle according to claim 3, being substantially free
from a dopant element.

17. The tin oxide particle according to claim 4, being substantially free
from a dopant element.

18. The tin oxide particle according to claim 2, which is for use as a
negative electrode active material or a raw material of a positive
electrode active material for lithium secondary batteries.

19. The tin oxide particle according to claim 3, which is for use as a
negative electrode active material or a raw material of a positive
electrode active material for lithium secondary batteries.

20. The tin oxide particle according to claim 4, which is for use as a
negative electrode active material or a raw material of a positive
electrode active material for lithium secondary batteries.

Description:

TECHNICAL FIELD

[0001] This invention relates to a novel tin oxide particle and a process
for producing the same.

BACKGROUND ART

[0002] It is known that a non-electroconductive material, such as
plastics, may be made electroconductive by the addition of an
electrically conductive powder. Examples of known electroconductive
powders include metal powders, carbon black, and tin oxide doped with
antimony or a like dopant. Addition of metal powder or carbon black to
plastics makes the plastics black, which can limit the utility of the
plastics. Addition of tin oxide doped with antimony, etc. makes plastics
bluish black, which can limit the utility of the plastics as with the
case of adding carbon black. In addition, using antimony involves the
problem of environmental burdens. Hence, various studies have been
reported on tin oxide free from a dopant, such as antimony.

[0003] Patent literature 1 (see below) describes alkali-stabilized tin
oxide sol having a particle size of 30 nm or smaller and containing
tetramethylammonium hydroxide in an NH3 to SnO2 molar ratio of 0.01
to 0.3. The tin oxide sol is obtained by adding tetramethylammonium
hydroxide to an alkaline tin oxide sol having a tin oxide concentration
of 15 wt % or less in terms of SnO2, followed by concentration.

[0004] Patent literature 2 teaches an alternative process for preparing
tin oxide sol comprising adding tin to 0.1 to 8 N hydrochloric acid in an
HCl to Sn molar ratio of 0.5 to 1 and then adding thereto a hydrogen
peroxide solution. According to the disclosure, the resulting tin oxide
particles have an average particle size of 5 to 100 nm.

[0005] Patent literature 3 proposes particles, which are not tin oxide
particles but a precursor for producing tin oxide particles, having a
flaky shape and containing 60 to 88 wt % of Sn and 1 to 15 wt % of
organic matter in terms of carbon. The precursor particles disclosed show
a sharp peak at about 9° in XRD. According to the disclosure, this
peak is attributed to the flaky shape of the particles.

[0006] However, the tin oxide particles produced by the above described
techniques are not regarded as being sufficient in electroconductivity
and transparency when formed into film.

[0007] Apart from the above techniques, tin (II) oxide having an
orthorhombic crystal structure with lattice constants of a=0.5 nm,
b=0.572 nm, and c=0.1112 nm is reported in non-patent literature 1 (see
below). A report on the space group of this tin oxide is also found in
the same literature. Based on these data, the inventors of the present
invention calculated the X-ray diffraction peak of the tin oxide and
found that the peak is at about 28°. It was also found that the
tin oxide shows a peak ascribed to the internal structure at about
60° or greater. According to the literature, however, the tin
oxide is unstable and ready to change to another structure. The
literature is silent on the electroconductivity or transparency of the
tin oxide.

[0012] An object of the invention is to provide tin oxide particles free
from the drawbacks of the above described conventional techniques and a
process for producing such tin oxide particles.

Solution to Problem

[0013] The invention provides a tin oxide particle having at least two
diffraction peaks at 2θ (deg) of 9±1° and 28±1°
in XRD measurement by Cu/Kα radiation.

[0014] The invention also provides a suitable process for producing the
tin oxide particle. The process includes mixing an aqueous solution
containing tin (II) and an organic compound having a hydroxyl group with
an alkali and heating.

[0015] The invention also provides another suitable process for producing
the tin oxide particle. The process includes mixing an alkali into an
aqueous solution containing tin (II) in a heated condition in such an
amount as to produce a molar quantity of OH.sup.- 0.1 to 1.6 times the
molar quantity of the tin (II).

Advantageous Effects of Invention

[0016] The invention provides tin oxide particles excellent in
transparency and electroconductivity when formed into film.

[0021]FIG. 5 depicts a graph showing charge/discharge curves of a lithium
secondary battery having a positive electrode active material prepared by
mixing the tin oxide particles of Example 1 and lithium nitrate and
firing the mixture at 400° C. in the atmosphere.

[0023] The present invention will be described based on its preferred
embodiments. The tin oxide particle of the invention is electroconductive
and has a structure characterized by at least two main diffraction peaks
at 2θ (deg) of 9±1° and 28±1° in XRD
measurement by Cu/Kα radiation. All the types of tin oxide hitherto
known, including SnO2 and SnO, do not show diffraction peaks either
of the two angles mentioned above. In other words, tin oxide particles
having diffraction peaks at these angles are unknown. The tin oxide
particle of the invention is completely novel.

[0024] The increased electroconductivity of conventionally known
electroconductive tin oxide is generally obtained by doping tetravalent
tin with a dopant element, such as antimony, niobium, or tantalum. In
contrast, the increased electroconductivity of the tin oxide according to
the invention owes to control of the crystal structure of tin oxide. The
technique of the invention allows for increasing the electroconductivity
of tin oxide particles while overcoming the drawbacks of using a dopant
element, such as economical disadvantage and large environmental burden.
Tin oxide having only divalent tin is, while electroconductive,
black-colored and therefore unable to be used in applications requiring
transparency, such as a transparent electroconductive film. Tin oxide
having only tetravalent tin is unable to have increased
electroconductivity over that of tin oxide having only divalent tin. In
contrast, the tin oxide particles of the invention have a whitish color,
which allows use as a transparent electroconductive film, and exhibits
high electroconductivity, which makes it feasible to provide the
transparent electroconductive film with increased electroconductivity.

[0025] The tin oxide particle of the invention further shows diffraction
peaks at 2θ (deg) of 19±1°, 48±1°, and
59±1° in addition to the two peaks mentioned above. The peaks
at 9±1° and 28±1° are the main peaks with higher
intensity than others. The present inventors presume that the tin oxide
of the invention has such a layered crystal structure that shows spatial
fluctuations triggered by a long period structure and the like existing
in a crystal plane. Based on this presumption, given that the peak at
2θ=9±1° is assigned to the reflection on the (001) plane,
the peak at 19±1° is assigned to the reflection on the (002)
plane, the peak at 28±1° is assigned to the reflection on the
(003) plane, the peak at 48±1° is assigned to the reflection on
the (005) plane, and the peak at 59±°1 is assigned to the
reflection on the (006) plane. It is concluded that all these peaks are
attributed to systematic reflection, namely first to sixth order
systematic reflection. The peak attributed to the reflection on the (004)
plane is too weak compared with the other peaks to be observed in
practice.

[0026] The spacing of crystal lattice planes of the tin oxide particle of
the invention corresponding to the first order systematic reflection was
found to be 0.94 to 0.95 nm with a standard deviation less than
1×10-4 nm. This strongly implies that the tin oxide particle
of the invention has the above stated layered crystal structure.

[0027] The tin oxide particle of the invention is also characterized in
that the peaks at 9±1° and 28±1° are very sharp.
Sharpness of a peak reflects the level of crystallinity. That is, the tin
oxide particle of the invention has high crystallinity. Regarding to the
systematic reflection as referred to above, the tin oxide particle of the
invention does not show reflection of higher order than sixth order in
XRD despite of its high crystallinity. This remarkable result of
observation is another characteristic of the tin oxide particle of the
invention.

[0028] The tin oxide particle of the invention is also characterized by
its thermal behavior in a reducing atmosphere. More specifically, when
the tin oxide particle is heated in a nitrogen atmosphere containing 1%
to 4% hydrogen at 400° C. for 2 hours, it comes to exhibit the
peak of metallic Sn that is not observed before the heating in XRD. In
some cases, peaks of SnO2 and SnO are observed after the heating. In
contrast, SnO2 or SnO does not show change of peaks in XRD even when
heated under the same conditions. Thus, on being heated in a reducing
atmosphere, the tin oxide particle of the invention displays such unique
properties that part of tin is reduced to metallic tin and that SnO2
and SnO increase in crystallinity, resulting in coexistence of
zero-valent Sn, divalent Sn, and tetravalent Sn.

[0029] XRD is carried out using a powder X-ray diffractometer RINT-TTRIII
from Rigaku Corp. Powder prepared in the manner, e.g., of Example 1 is
put in a dedicated glass-made holder and analyzed by XRD under the
following conditions:

[0030] It is preferred for the tin oxide particle of the invention to
contain only tin as a metal element and only oxygen (in some cases oxygen
and hydrogen) as other elements and to be substantially free from a
dopant element, namely, of non-doped type. For tin oxide particles to be
non-doped is advantageous in that highly electroconductive tin oxide
particles are obtained without using various dopant elements that are
expensive and economically noncompetitive or impose great environmental
burdens. Examples of dopant elements include those conventionally used in
the art, such as Nb, Ta, Sb, W, P, Ni, and Bi. By the term "substantially
free from" as used herein is meant that intentional addition of a dopant
element is excluded. Unavoidable incorporation of a trace amount of a
dopant element during the production process of the tin oxide particles
is therefore allowable.

[0031] As stated, the tin oxide particle of the invention is preferably
free from a dopant element. Nevertheless, a dopant element may be present
in some specific applications. In the cases where the tin oxide particle
contains a dopant element, the content of the dopant element is
preferably 0.01 to 20 mol %, more preferably 0.05 to 15 mol %, relative
to the total tin from the viewpoint of enhancing the electroconductivity
without impairing cost economy. The dopant elements that may be used in
such cases include one or more of the above recited elements.

[0032] The tin oxide particles of the invention preferably have an average
primary particle size of 1 to 5000 nm, more preferably 3 to 3000 nm, even
more preferably 3 to 1000 nm, as observed using a scanning electron
microscope (SEM).

[0033] The tin oxide particles of the invention are also characterized by
high electroconductivity, which is specifically represented by such low
resistance as a powder volume resistivity of 105 Ω-cm or less,
preferably 104 Ω-cm or less, more preferably 103
Ω-cm or less, under a 500 kgf load. The method of measuring powder
volume resistivity will be described later.

[0034] The tin oxide particles of the invention exhibit high transparency
when formed into film. For example, a film having a thickness of 2 to 3
μm and containing the tin oxide particles in an amount of 30% to 80%
exhibits very high transparency as having a total transmission of 85% or
more, preferably 90% or more. The method for film formation will be
described in detail in Examples hereinafter given.

[0035] A preferred process for producing the tin oxide particles of the
invention will then be described. In the process, divalent tin (tin (II))
is used as a starting material. The divalent tin is dissolved in water
together with an organic compound having a hydroxyl group to make a mixed
aqueous solution, and the mixed aqueous solution is, while being in a
heated condition, mixed with an alkali. Each step is described below in
detail.

[0036] A water soluble compound of tin (II) is provided as a starting
material. Such a water soluble compound is exemplified by tin (II)
chloride. The tin (II) ion concentration in the mixed aqueous solution
may range from 0.01 to 3 mol/L, preferably 0.05 to 1.5 mol/L. Although
using a tin (IV) compound as a starting material is conceivable, the
inventors' investigations have revealed that tin (II) is readier to
provide a desired oxide than tin (IV). Accordingly, tin (II) is used as a
starting material in the process of the invention.

[0037] Separately from the tin (II) compound, an organic compound having a
hydroxyl group is provided. Such an organic compound may be a low
molecular compound or a high molecular compound. Examples of a low
molecular, hydroxyl-containing organic compound include monohydric
alcohols that may be aliphatic, alicyclic, or aromatic. Examples of
aliphatic monohydric alcohols include monohydric alcohols having 1 to 6
carbon atoms, such as methanol, ethanol, n-butanol, and n-hexanol.
Examples of alicyclic monohydric alcohols are cyclohexanol and terpineol.
Examples of aromatic monohydric alcohols include benzyl alcohol.

[0038] The high molecular, hydroxyl-containing organic compound is
exemplified by polyvinyl alcohols and polyols. The polyvinyl alcohols may
be unmodified polyvinyl alcohol per se or modified polyvinyl alcohols.
The polyvinyl alcohol may be either completely saponified or partially
saponified (degree of saponification=80% to 90%). Examples of modified
polyvinyl alcohols include carboxyl-modified, alkyl-modified,
acetoacetyl-modified, acrylic acid-modified, methacrylic acid-modified,
pyrrolidone-modified, vinylidene-modified, and silanol-modified polyvinyl
alcohols. It is preferred to use a polyvinyl alcohol (--CH(OH)CH2--)n
having an average degree of polymerization (n) of 200 to 30000, more
preferably 500 to 10000. The degree of polymerization can be measured by
size exclusion chromatography (SEC). Examples of the polyols include
ethylene glycol, diethylene glycol, triethylene glycol, polyethylene
glycol, polypropylene glycol, propanediol, butanediol, pentanediol,
hexanediol, glycerol, hexanetriol, butanetriol, and
3-methylpentane-1,3,5-triol. Cellosolves, such as methoxyethanol,
ethoxyethanol, propoxyethanol, and buthoxyethanol; and carbitols, such as
methoxymethoxyethanol, ethoxyethoxyethanol, propoxyethoxyethanol, and
butoxyethoxyethanol, are also useful.

[0039] In using a monohydric alcohol as the organic compound, the
concentration of the hydroxyl-containing organic compound in the mixed
aqueous solution preferably ranges from 0.005% to 30%, more preferably
from 0.01% to 10%, by weight. Within that range, the effects of the
hydroxyl-containing organic compound are fully achieved; inconveniences
such as thickening hardly occur; and desired tin oxide particles having a
uniform particle size are successfully obtained. For the same reasons, in
using a high molecular hydroxyl-containing organic compound, the
concentration of the organic compound is preferably 0.005% to 10%, more
preferably 0.01 to 5%, by weight.

[0040] The ratio of tin (II) to hydroxyl-containing organic compound in
the mixed aqueous solution in terms of Sn to OH molar ratio is preferably
0.01 to 150, more preferably 0.03 to 75. Within that range, unreacted Sn
ions are less likely to remain in the solution, and SnO2 or tin (II)
hydroxide [Sn3O2(OH)2], which are by-products, hardly
precipitate.

[0041] A mixed aqueous solution containing tin (II) and the
hydroxyl-containing organic compound is thus prepared. The mixed aqueous
solution is then heated. The heating temperature is preferably 50°
to 105° C., more preferably 70° to 100° C. Within
that temperature range, desired tin oxide particles are obtained without
need to use a pressure-resistant apparatus, such as an autoclave, while
preventing unintentional production of SnO or SnO2.

[0042] To the mixed aqueous solution in a condition heated to a
temperature within the range recited above is added an alkali (basic
substance). Tin (II) is neutralized by this operation. Examples of the
alkali include alkali metal hydroxides, such as sodium hydroxide and
potassium hydroxide; alkaline earth metal hydroxides, such as magnesium
hydroxide; carbonates, such as NaHCO3 and NH4HCO3; and
ammonia. The pH of the aqueous alkali solution to be added is preferably
such that the mixed aqueous solution after the alkali addition may have a
pH of 2 to 9, more preferably 2.5 to 7. When the pH of the mixed aqueous
solution is in the range recited, desired tin oxide particles are
obtained in a single phase.

[0043] It is preferred that the addition of the aqueous alkali solution to
the mixed aqueous solution of tin (II) oxide and the hydroxyl-containing
organic compound be slow, taking a predetermined time. When the aqueous
alkali solution is added all at once, care should be taken because such
way of addition may fail to produce desired tin oxide particles. When the
aqueous alkali solution is added slowly, it is recommended to adjust the
rate of addition so that the pH of the mixed aqueous solution may be kept
within the range recited above.

[0044] Desired tin oxide particles thus form in the liquid. There can be
tin oxyhydroxide as a by-product in the liquid. It is therefore desirable
to add hydrogen peroxide to the liquid for the purpose of removing the
by-product. Addition of hydrogen peroxide accelerates oxidation of tin
oxyhydroxide to produce tin dioxide. Because tin dioxide is produced in
the form of fine particles, it can be separated by water elutriation
making use of the difference in settling velocity of particles. In water
elutriation, the desired tin oxide particles are sediment, while
SnO2 to be removed is in the supernatant. Because SnO2 is
dispersible under an alkaline condition, classification efficiency by
water elutriation may be increased by adjusting the pH of the liquid to 8
to less than 11 with, e.g., NH4OH and then highly dispersing
SnO2 by the use of a high-speed agitator or by ultrasonic
irradiation prior to the water elutriation. To control the oxidation of
the tin oxyhydroxide, hydrogen peroxide is preferably added in the form
of a diluted aqueous solution having a predetermined concentration. From
this viewpoint, the diluted hydrogen peroxide solution preferably has a
concentration of about 1% to 15% by weight.

[0045] The thus collected tin oxide particles may easily be freed of
impurities by, for example, repulping with water. To sufficiently remove
impurities, repulping with water is preferably performed until the
electroconductivity of the dispersing medium (water) reduced to 2000
μS or less, more preferably 1000 μS or less.

[0046] A dispersion of the tin oxide particles having been purified by
repulping until the dispersing medium has a prescribed reduced
electroconductivity is then subjected to a disagglomeration operation to
provide tin oxide sol. The disagglomeration operation may be implemented
by the use of, for example, a media mill, e.g., a bead mill. It is
preferred to carry out the disagglomeration operation in the presence of
a pH adjustor of various kinds so that tin oxide particles close to
monodisperse may be obtained. A pH adjustor may be added after the
disagglomeration. A pH adjustor capable of adjusting the pH of the
dispersion to 3 to 10, preferably 3 to 6, is preferably used. Examples of
such a pH adjustor include acids, such as inorganic acids (e.g.,
hydrochloric acid, sulfuric acid, and nitric acid) and carboxylic acids
(e.g., acetic acid and propionic acid), and alkalis, such as aqueous
ammonia and organic amines (e.g., ethanolamine).

[0047] As a result of the disagglomeration operation, tin oxide sol having
water as a dispersing medium is obtained. The tin oxide sol thus obtained
is a transparent dispersion having high storage stability. The tin oxide
sol preferably contains the tin oxide particle in a concentration of 0.1%
to 50%, more preferably 1% to 40%, by weight. In the tin oxide sol the
tin oxide particles are highly dispersed.

[0048] The above described process, in which tin oxide is produced in a
liquid phase (water), easily produces tin oxide sol with high
dispersibility and low agglomeration as compared with the conventional
processes in which tin oxide obtained by firing is pulverized and then
slurried into sol.

[0049] The tin oxide particles of the invention may be dispersed in an
organic solvent to prepare a transparent monodisperse dispersion. The
dispersing may be carried out using, for example, a bead mill or a paint
shaker. Useful organic solvents include polyhydric alcohols, monohydric
alcohols, cellosolves, carbitols, ketones, and mixtures thereof. The
concentration of the tin oxide particles in the transparent dispersion is
preferably 0.1% to 50%, more preferably 1% to 40%, by weight. The
transparent dispersion has high storage stability. The transparent
dispersion may serve as, for example, an ink material by addition of a
binder.

[0051] To further increase the electroconductivity of the tin oxide
particles of the invention, it has been turned out effective to thermally
treat the tin oxide particles obtained through the above described
process. The heat treatment is conducted in an oxygen-containing
atmosphere, such as the atmosphere. The heat treatment is preferably
carried out at 60° to 350° C., more preferably 120°
to 300° C., for 0.5 to 24 hours, more preferably 1 to 10 hours.
The heat treatment causes no change in the crystal structure of the tin
oxide particle, as will be demonstrated by comparing the XRD results of
Example 1 (FIG. 1) and those of Example 18 (FIG. 6).

[0052] The above described process employs a hydroxyl-containing organic
compound in the synthesis of the tin oxide as desired. Alternatively, the
hydroxyl-containing organic compound may not be used. In this alternative
process, an alkali (base) is mixed into an aqueous solution containing
tin (II) in a heated condition preferably in such an amount as to produce
a molar quantity of OH.sup.- 0.1 to 1.6 times, more preferably 0.3 to 1.4
times, the molar quantity of the tin (II). The aqueous solution
containing tin (II) that can be used in this process may be the same as
used in the aforementioned process. This also applies to the alkali.

[0053] In the alternative process, the aqueous solution containing tin
(II), which does not contain a hydroxyl-containing organic compound, is
heated preferably to 50° to 105° C., more preferably
70° to 100° C. An alkali is added to the aqueous solution
in a condition heated to a temperature in that range. By this operation,
divalent tin is neutralized. In the present process, the amount of the
alkali to be added for tin (II) neutralization is significant. That is,
it is necessary to add the above-specified amount of the alkali. This
amount of concern is smaller than the alkali that is used in the
aforementioned process in which a hydroxyl-containing organic compound is
used in combination. If the alkali (base) is added in an amount that
produces a quantity of OH-- exceeding 1.6 times the molar quantity of tin
(II), an inconvenience results, in which black plate-shaped coarse SnO
particles are produced.

[0054] Similarly to the process using the hydroxyl-containing organic
compound in combination, addition of the aqueous alkali solution to the
tin (II) aqueous solution is preferably conducted slowly over a
prescribed period of time. In this case, it is preferred to maintain the
pH of the aqueous tin (II) solution at 2 to 9, more preferably 2.5 to 7.

[0055] Thus, tin oxide particles as desired are produced in the liquid.
The resulting liquid is treated in the same manner as in the process
using the hydroxyl-containing organic compound.

[0056] The tin oxide particles thus obtained are useful in broad
applications with their high electroconductivity being taken advantage
of. The applications include charging rollers, photoreceptor drums,
toners, electrostatic brushes, and the like of printers or copiers; flat
panel displays, CRTs, Braun tubes, and the like; coatings, inks,
emulsions, and the like. With the layered crystal structure being taken
advantage of, the tin oxide particles are also useful as a raw material
of a positive electrode active material or as a negative electrode active
material of lithium secondary batteries or a gas fixing material. In
using the tin oxide particles as a raw material of a positive electrode
active material, the tin oxide particles are mixed with a
lithium-containing compound (e.g., lithium nitrate), and the mixture is
fired in the atmosphere to yield a lithium-tin double oxide, which can be
used as a positive electrode active material.

EXAMPLES

[0057] The invention will now be illustrated in greater detail with
reference to Examples, but it should be understood that the invention is
not construed as being limited thereto. Unless otherwise noted, all the
percents are by weight.

Example 1

[0058] In 418 g of pure water was dissolved 4.51 g of sodium hydroxide to
prepare an aqueous alkali solution for neutralization. Separately, 0.5 g
of polyvinyl alcohol (hereinafter abbreviated as PVA) partially
saponified and having an average degree of polymerization n=1500 to 1800
was put in a 200 ml beaker previously containing 100 g pure water and
dissolved by heating to 60° C. to prepare an aqueous PVA solution.
Separately, 14.97 g of tin dichloride dihydrate was dissolved in 383 g of
pure water in a beaker to prepare an aqueous tin solution. The whole
amount of the aqueous PVA solution was added to the aqueous tin solution,
and the system was thoroughly mixed up to obtain a mother liquor.

[0059] The mother liquor was heated to 90° C. while stirring with a
paddle stirrer, and the whole amount of the above prepared aqueous alkali
solution was fed thereto over 90 minutes using a tube pump (feed rate:
about 5 ml/min). During the alkali addition, the mother liquor had a pH
of 3 to 4. After completion of the addition, the system was aged for 5
minutes. Then, a solution of 7.5 g of 30% hydrogen peroxide in 30 g of
pure water was added to the mother liquor at a rate of 5 ml/min, followed
by aging for 5 minutes to give desired tin oxide sol. The pH of the sol
was between 2 and 3.

[0060] The sol was filtered through filter paper (Advantec 5C). The filter
cake was washed by pouring 1 L pure water. The resulting cake was
repulped in pure water, filtered, and washed by pouring water again.
These operations were repeated three times. The thus washed cake was
dried in the atmosphere in a hot air drier set at 120° C. for 10
hours, and the solid was disagglomerated in an agate mortar and
classified using an SUS mesh having an opening size of 75 μm. The
results of elementary analysis of the resulting powder are shown in Table
1. The elementary analysis was carried out by using ICP (SPS-3000, from
SII NanoTechnology Inc.) for tin, silicon, and iron, a gas analyzer
(EMGA-620, from Horiba, Ltd.) for oxygen, and a gas analyzer (EMIA-920V,
from Horiba, Ltd.) for carbon. Absorption spectrometry (turbidimetry
using silver nitrate) was used for chlorine (cf. Applied Inorganic
Colorimetry Editorial Committee, Muki Oyo Hishoku Bunseki-2, Kyoritsu
Publ.).

[0061] The powder was subjected to XRD according to the procedure
described above. The crystal lattice spacing, powder resistivity, and a
total transmission of a film were determined by the methods described
below. The results obtained are shown in FIG. 1 and Table 2.

(1) Crystal Lattice Spacing

[0062] Interpreting the above described five peaks observed from
2θ=9° to 59° as being attributed to the (001), (002),
(003), (005), and (006) planes in sequence, the lattice spacing was
determined by the least square method.

(2) Powder Resistivity

[0063] The tin oxide particles were compressed under a pressure of 500 kgf
to make a sample. The resistivity of the sample was measured by the
four-prove resistance method using Lorest PAPD-41 from Mitsubishi
Chemical Corp.

(3) Total Transmission of Film

[0064] The tin oxide particles weighing 7.4 g and 6.4 g of a commercially
available acrylic resin were added to 10 g of a toluene/butanol (=7:3 by
weight) mixed solvent and dispersed therein with beads in a paint shaker.
The resulting dispersion was applied to a PET film and air dried for 1
hour to form a transparent film, the thickness of which was found to be 2
μm as observed using an electron microscope. The total transmission of
the film was determined using a transmission measuring instrument
NDH-1001DP from Nippon Denshoku Industries Co., Ltd.

[0065] In order to examine the internal crystal structure of the tin oxide
powder obtained in Example 1, the tin oxide was subjected to X-ray
diffractometry by using synchrotron radiation at Spring-8, a largest
scale radiant light facility by Japan Synchrotron Radiation Research
Institute. The wavelength of the X-rays used in the measurement was
0.0501326 nm. The sample to be analyzed was packed in a glass capillary
tube loosely so as not to be oriented in a certain direction. X-Ray
diffraction lines were recorded using a Debye-Scherrer camera and
converted to intensity vs. 2θ. The results obtained are shown in
FIG. 2, in which the peaks indicated by downward pointing triangles
correspond to the peaks indicated by circles in FIG. 1.

Example 2

[0066] Tin oxide particles were obtained in the same manner as in Example
1, except for using 0.5 g of ethanol in place of PVA. The resulting tin
oxide particles were analyzed in the same manner as in Example 1. The
results obtained are shown in FIG. 1 and Table 2.

Example 3

[0067] Tin oxide particles were obtained in the same manner as in Example
1, except for using 0.5 g of n-butanol in place of PVA. The resulting tin
oxide particles were analyzed in the same manner as in Example 1. The
results obtained are shown in Table 2.

Example 4

[0068] Tin oxide particles were obtained in the same manner as in Example
1, except for using 0.5 g of hexanol in place of PVA. The resulting tin
oxide particles were analyzed in the same manner as in Example 1. The
results obtained are shown in FIG. 1 and Table 2.

Example 5

[0069] Tin oxide particles were obtained in the same manner as in Example
1, except for using 0.5 g of benzyl alcohol in place of PVA. The
resulting tin oxide particles were analyzed in the same manner as in
Example 1. The results obtained are shown in FIG. 1 and Table 2.

Comparative Example 1

[0070] Tin oxide particles were obtained in the same manner as in Example
1, except that PVA was not used. The resulting tin oxide particles were
analyzed in the same manner as in Example 1. The results obtained are
shown in FIG. 3 and Table 2.

Comparative Example 2

[0071] Tin oxide particles were obtained in the same manner as in Example
1, except that the mother liquor was not heated, i.e., the reaction was
conducted at room temperature (25° C.). The resulting tin oxide
particles were analyzed in the same manner as in Example 1. The results
obtained are shown in FIG. 3 and Table 2. The tin oxide particles were
also subjected to elementary analysis in the same manner as previously
described. The results are shown in Table 3.

[0072] As is apparent from the results shown in FIG. 1, all of the tin
oxide particles obtained in Examples 1 through 5 show five peaks at the
same positions. In contrast, the tin oxide particles obtained in
Comparative Examples show only the diffraction peaks of SnO2
(Comparative Example 2) or the diffraction peaks of SnO2 and other
diffraction peaks (Comparative Example 1) as depicted in FIG. 3. As is
apparent from the results in Table 2, it is also proved that the tin
oxide particles obtained in each Example exhibit higher
electroconductivity than those of Comparative Examples. The tin oxide
particles of each Example are also proved to provide a film with high
transparency as compared with a film of each Comparative Example.

[0073] Comparing between FIGS. 1 and 2 shows that the peaks observed in
the XRD shown in FIG. 1 are also observed in FIG. 2. Furthermore, FIG. 2
demonstrates peaks other than those observed in the XRD shown in FIG. 1,
which suggests that the tin oxide crystal obtained in Example 1 has an
internal structure.

Example 6

[0074] Example 6 is to verify the usefulness of the tin oxide particles of
the invention as a negative electrode active material of a lithium
secondary battery.

[0075] The tin oxide particles of Example 1 weighing 2.85 g, 0.15 g of
acetylene black, and 0.33 g of polyvinylidene fluoride were mixed, and 3
g of N-methyl-2-pyrrolidinone was added thereto, followed by mixing in a
defoaming mixer (from Thinky Corp.) to prepare a slurry. The slurry was
applied to a side of 18 μm-thick copper foil and dried at 120°
C. The coated copper foil was cut to a width of 6 cm, and a pressure of 2
ton was applied thereto using a roll press. A 14 mm diameter circle was
stamped out from the foil, followed by drying in vacuo at 120° C.
overnight to make a negative electrode. The amount of the tin oxide
particles (active material) of the negative electrode was 6 mg/cm2.
A size 2032 coin cell was assembled in an argon atmosphere in a glove box
using the resulting negative electrode, Li foil as a counter electrode,
and, as an electrolyte, a 1 mol/L solution of LiPF6 in a 1:1 (by
volume) mixed solvent of ethylene carbonate and diethyl carbonate. The
resulting coin cell was subjected to charge/discharge testing under the
following conditions. The cell was charged at a fixed current rate (0.175
mA/cm2) up to a voltage of 0.0 V (vs. Li.sup.+/Li) and then at a
constant voltage of 0.0 V until the current density reduced to 0.035
mA/cm2. The cell was discharged at a constant current rate (0.175
mA/cm2) to a voltage of 2.5 V (vs. Li.sup.+/Li). The results are
shown in FIG. 4. The results in FIG. 4 prove that the tin oxide particles
of the invention have a charge/discharge capacity and are useful as a
negative electrode active material of a lithium secondary battery.

Example 7

[0076] Example 7 is to verify the usefulness of the tin oxide particles of
the invention as a raw material of a positive electrode active material
of a lithium secondary battery.

[0077] The tin oxide particles obtained in Example 1 weighing 6.00 g and
LiNO3 weighing 2.61 g were thoroughly mixed in a mortar. The mixture
was packed into an aluminum boat and fired in the atmosphere at
400° C. for 5 hours to prepare a positive electrode active
material comprising a lithium-tin double oxide. The positive electrode
active material weighing 2.85 g, 0.15 g of acetylene black, and 0.33 g of
polyvinylidene fluoride were mixed, and 3 g of N-methyl-2-pyrrolidinone
was added thereto, followed by mixing in a defoaming mixer (from Thinky
Corp.) to prepare a slurry. The slurry was applied to a side of 18
μm-thick aluminum foil and dried at 120° C. The aluminum foil
was cut to a width of 6 cm, and a pressure of 2 ton was applied to the
cut foil using a roll press. A 14 mm diameter circle was stamped out from
the foil, followed by drying in vacuo at 120° C. overnight to make
a positive electrode. The amount of the active material of the positive
electrode was 6 mg/cm2. A size 2032 coin cell was assembled in an
argon atmosphere in a glove box using the resulting positive electrode,
Li foil as a counter electrode, and, as an electrolyte, a 1 mol/L
solution of LiPF6 in a 1:1 (by volume) mixed solvent of ethylene
carbonate and diethyl carbonate. The resulting coin cell was subjected to
charge/discharge testing under the following conditions. The cell was
charged at a fixed current rate (0.175 mA/cm2) up to a voltage of
4.8 V (vs. Li.sup.+/Li) and then at a constant voltage of 4.8 V until the
current density reduced to 0.035 mA/cm2. The cell was discharged at
a constant current rate (0.175 mA/cm2) to a voltage of 2.7 V (vs.
Li.sup.+/Li). The results are shown in FIG. 5. The results in FIG. 5
prove that the tin oxide particles of the invention have a
charge/discharge capacity and are useful as a positive electrode active
material of a lithium secondary battery.

Example 8

[0078] A transparent dispersion was prepared using the tin oxide particles
of the invention and an organic solvent as a dispersing medium as
follows:

[0079] In a 50 ml-volume hermetic container made of polypropylene were put
1.57 g of the tin oxide particles obtained in Example 1, 18 g of ethylene
glycol, and 140 g of 0.1 mm diameter zirconia beads and shaken using a
paint shaker for 3 hours to disagglomerate the particles. After the
disagglomeration operation, the beads were separated by filtration under
reduced pressure to give a beige transparent dispersion. The dispersion
retained the highly disperse state without settling the particles even
after kept at ambient temperature for one month. A glassy solid remained
on drying the dispersion at 200° C., and the residual solids
content was found to be 8 wt %.

Example 9

[0080] A transparent dispersion was prepared using the tin oxide particles
of the invention and water as a dispersing medium as follows:

[0081] In a 50 ml volume polypropylene hermetic container were put 1.39 g
of the tin oxide particles obtained in Example 1, 16 g of water, and 140
g of 0.1 mm diameter zirconia beads and shaken using a paint shaker for 3
hours to disagglomerate the particles. After the disagglomeration
operation, the beads were separated by filtration under reduced pressure.
The resulting dispersion had a pH of 5.4. The dispersion was adjusted to
pH 3.0 by addition of a small amount of acetic acid to give a beige
transparent dispersion. The dispersion retained the highly disperse state
without settling the particles even after kept at ambient temperature for
one month. A glassy solid remained on drying the dispersion at
200° C., and the residual solids content was found to be 7 wt %.

Example 10

[0082] Tin oxide particles were obtained in the same manner as in Example
1, except for using 12.58 g of anhydrous tin (II) chloride in place of
14.97 g of tin (II) chloride dihydrate. The resulting tin oxide particles
were analyzed by XRD in the same manner as in Example 1. The results are
shown in FIG. 6.

Example 11

[0083] Tin oxide particles were obtained in the same manner as in Example
10, except for increasing the amount of the PVA to 5.0 g. The resulting
tin oxide particles were analyzed by XRD in the same manner as in Example
1. The results are shown in FIG. 6.

Example 12

[0084] Tin oxide particles were obtained in the same manner as in Example
11, except for using partially saponified polyvinyl alcohol having an
average degree of polymerization n of 500 as PVA. The resulting tin oxide
particles were analyzed by XRD in the same manner as in Example 1. The
results are shown in FIG. 6.

Example 13

[0085] Tin oxide particles were obtained in the same manner as in Example
11, except for using completely saponified polyvinyl alcohol having an
average degree of polymerization n of 400 to 600 as PVA. The resulting
tin oxide particles were analyzed by XRD in the same manner as in Example
1. The results are shown in FIG. 6.

Example 14

[0086] Tin oxide particles were obtained in the same manner as in Example
11, except for using of completely saponified polyvinyl alcohol having an
average degree of polymerization n of 900 to 1100 as PVA. The resulting
tin oxide particles were analyzed by XRD in the same manner as in Example
1. The results are shown in FIG. 6.

Example 15

[0087] Tin oxide particles were obtained in the same manner as in Example
10, except for using no PVA and reducing the amount of sodium hydroxide
to 2.65 g. The resulting tin oxide particles were analyzed by XRD in the
same manner as in Example 1. The results are shown in FIG. 6.

Example 16

[0088] Tantalum-doped tin oxide particles were obtained in the same manner
as in Example 13, except for replacing the aqueous tin solution with a
mixed aqueous solution containing tin and tantalum that was prepared by
dissolving 12.57 g of anhydrous tin (II) chloride and 0.016 g of tantalum
pentachloride. The resulting particles were dried in the atmosphere at
120° C. for 10 hours and classified using an SUS mesh having an
opening size of 75 μm. The resulting powder was subjected to XRD in
the same manner as in Example 1. Additionally, the crystal lattice
spacing, powder resistivity, and a total transmission of a film were
determined in the same manner as in Example 1. The results obtained are
shown in FIG. 6 and Table 4.

Example 17

[0089] The powder obtained in Example 16 was fired in the atmosphere at
300° C. for 2 hours in an electric oven. The fired powder was
subjected to XRD in the same manner as in Example 1. Additionally, the
crystal lattice spacing, powder resistivity, and a total transmission of
a film were determined in the same manner as in Example 1. The results
obtained are shown in FIG. 6 and Table 4.

Example 18

[0090] Example 18 was carried out for comparison with Example 17. The
powder obtained in Example 1 was fired in the atmosphere at 300°
C. for 2 hours in an electric oven. The fired powder was subjected to XRD
in the same manner as in Example 1. Additionally, the crystal lattice
spacing, powder resistivity, and a total transmission of a film were
determined in the same manner as in Example 1. The results obtained are
shown in FIG. 6 and Table 4.

[0091] By comparing the powder resistivity of Example 18 shown in Table 4
with that of Example 1 shown in Table 2, it is seen that the tin oxide of
Example 18, which was obtained by heat treating the tin oxide of Example
1 at high temperature, exhibits higher electroconductivity. By comparing
the powder resistivity between Examples 16 and 17 shown in Table 4, it is
seen that the improvement in electroconductivity by the high temperature
treatment is further enhanced by doping with tantalum.